TRANSIENT SYNOVITIS
Most Common Cause of Hip Pain in Children | Self-Limiting | Must Exclude Septic Arthritis | Kocher Criteria Critical
KOCHER CRITERIA FOR SEPTIC ARTHRITIS RISK
Critical Must-Knows
- Most common cause of hip pain in children aged 3-8 years - self-limiting condition
- Kocher criteria differentiate from septic arthritis: fever over 38.5°C, non-weight bearing, ESR over 40, WCC over 12,000
- 0-1 criteria = 3% septic risk (observe), 4 criteria = 99% septic risk (urgent I&D)
- Hip aspiration is gold standard if doubt - WCC under 50,000 with under 75% PMN suggests transient synovitis
- Self-limiting - resolves in 7-10 days with rest and NSAIDs, no long-term sequelae
Examiner's Pearls
- "Always exclude septic arthritis first - it's an orthopaedic emergency
- "Kocher criteria are high-yield exam content - know all 4 criteria and probability for each
- "Hip aspiration: Septic arthritis typically over 50,000 WCC with over 75% PMN
- "Transient synovitis: WCC 5,000-15,000 with 40-50% PMN, culture negative
- "Post-viral history common in transient synovitis but not diagnostic
Clinical Imaging
Imaging Gallery
Critical Exam Concepts
Exclude Septic Arthritis First
Septic arthritis is orthopaedic emergency - must be excluded in every case. Use Kocher criteria: fever over 38.5°C, non-weight bearing, ESR over 40, WCC over 12,000. 4 criteria = 99% probability septic = urgent I&D. Never miss septic arthritis.
Kocher Criteria are Critical
Know all 4 criteria and probabilities: 0-1 = 3% (observe), 2 = 40% (consider aspiration), 3 = 93% (likely I&D), 4 = 99% (urgent I&D). CRP over 20 may be added as 5th criterion in some studies.
Hip Aspiration is Gold Standard
If doubt exists, aspirate. Septic: over 50,000 WCC, over 75% PMN, culture positive. Transient: 5,000-15,000 WCC, 40-50% PMN, culture negative. Low WCC does not completely exclude infection - clinical picture matters.
Self-Limiting Condition
Transient synovitis resolves in 7-10 days with rest and NSAIDs. No long-term sequelae. No need for antibiotics if diagnosis confirmed. Reassurance and symptomatic treatment sufficient.
Transient Synovitis vs Septic Arthritis
| Feature | Transient Synovitis | Septic Arthritis |
|---|---|---|
| Age | 3-8 years (peak) | Any age, often younger |
| Fever | Low-grade or absent | High fever over 38.5°C |
| Weight bearing | May weight bear with limp | Refuses all weight bearing |
| ESR | Normal or mildly elevated | Over 40mm/hr |
| WCC | Normal or mildly elevated | Over 12,000 |
| Aspiration WCC | 5,000-15,000 | Over 50,000 |
| Aspiration PMN | 40-50% | Over 75% |
| Treatment | Rest, NSAIDs, observe | Urgent I&D, IV antibiotics |
| Outcome | Resolves 7-10 days, no sequelae | Urgent treatment prevents joint destruction |
FENWKocher Criteria for Septic Arthritis
Memory Hook:FENW - Four criteria, if all present = 99% septic arthritis!
TRANSIENTTransient Synovitis Features
Memory Hook:TRANSIENT - it's temporary, resolves on its own!
SEPTICSeptic Arthritis Aspiration
Memory Hook:SEPTIC - high WCC, high PMN, positive culture, urgent treatment!
Overview and Epidemiology
Key Exam Concept
Differentiation is Key. The exam focus is almost entirely on distinguishing this benign condition from septic arthritis. You must demonstrate a safe, logical approach using Kocher criteria. Missing septic arthritis is a critical fail.
Epidemiology
- Peak age: 3-8 years (most common 4-6 years)
- Gender: Slight male predominance (1.5:1)
- Incidence: Most common cause of hip pain in children
- Seasonal: May follow viral illness (winter/spring)
- Recurrence: 5-15% may have recurrent episodes
Natural History
- Self-limiting: Resolves spontaneously in 7-10 days
- No long-term sequelae: No increased risk of Perthes or other hip pathology
- Recurrence: May recur but each episode resolves
- Prognosis: Excellent - complete resolution expected
Pathophysiology and Mechanisms
Pathogenesis of Transient Synovitis
Mechanism: Benign, self-limiting inflammation of the hip synovium (synovitis).
Etiology: Exact cause unknown, but widely accepted as:
- Post-viral: Often follows upper respiratory tract infection (1-2 weeks prior).
- Post-infectious: Immune-mediated response to recent infection.
- Non-bacterial: Joint fluid is sterile.
Process: Synovial inflammation leads to Joint effusion leads to Capsular distension leads to Pain and limitation of movement (especially internal rotation/abduction).
Classification Systems
Validated Prediction Rule
There is no classification for transient synovitis itself. The relevant "classification" is the Risk Stratification for Septic Arthritis using Kocher Criteria.
| Criteria Count | Septic Arthritis Probability | Recommended Action |
|---|---|---|
| 0 criteria | less than 0.2% | Observe |
| 1 criterion | 3% | Observe / Symptomatic treatment |
| 2 criteria | 40% | Make clinical judgment / Aspirate |
| 3 criteria | 93% | Urgent Aspiration |
| 4 criteria | 99% | Urgent Aspiration & I&D |
The 4 Criteria:
- Fever over 38.5°C
- Non-weight bearing on affected side
- ESR over 40 mm/hr
- WCC over 12,000 cells/mm³
Clinical Assessment
History
- Onset: Acute or subacute (hours to days)
- Pain: Hip/groin pain, may refer to thigh or knee
- Limp: Refusal to walk or antalgic limp
- Recent illness: URI common 1-2 weeks prior
- Systemic: Child usually appears well
Examination
- Vital signs: Usually afebrile or low-grade (under 38.5°C)
- Gait: Antalgic limp (short stance phase)
- ROM: Restriction in Internal Rotation and Abduction.
- Log roll: May be irritable but less severe than septic.
- Tenderness: Anterior joint line tenderness.
Always Exam the Knee
Referred Pain: Hip pathology in children frequently presents as knee pain (via obturator nerve). ALWAYS examine the hip in any child presenting with knee or thigh pain. A normal knee exam with an irritable hip suggests hip pathology.
Investigations
Essential Bloods
FBC: WCC usually normal or mildly elevated (under 12,000). Over 12,000 is a Kocher criterion. ESR: Usually normal or mild (under 40). Over 40 is a Kocher criterion. CRP: Usually normal (under 20).
Blood Culture: Not routine if low suspicion. Essential if febrile or septic concern.
Management Algorithm
Step-by-Step Management
- Calculate Kocher Score: Assess fever, weight-bearing, WCC, ESR.
- Low Risk (0-1): OBSERVE. Prescribe NSAIDs. Rest. Review in 48 hours.
- Moderate Risk (2): CONSIDER ASPIRATION. Or close observation if clinical picture benign.
- High Risk (3-4): URGENT ASPIRATION. If pus leads to Surgery. If unsure leads to Surgery.
Aspiration Thresholds:
- Transient Synovitis: WCC under 50,000, PMN under 75%, Gram stain negative.
- Septic Arthritis: WCC over 50,000, PMN over 75%, Gram stain positive.
Clinical judgment is required when results are equivocal. Aspiration is the only way to definitively rule out sepsis in high-risk cases.
Detailed Differential Diagnosis
Differentiating Perthes Disease
Perthes disease (Legg-Calvé-Perthes) is the main differential for a limping child in this age group (4-8 years).
| Feature | Transient Synovitis | Perthes Disease |
|---|---|---|
| Onset | Acute (days) | Insidious (weeks/months) |
| Pain | Constant, antalgic | Activity-related, often mild |
| ROM | Restricted in acute phase only | Chronic restriction (Abduction/IR) |
| X-ray | Normal / Effusion | Sclerosis / Fragmentation / Flattening |
| Systemic | Post-viral history | Well child, small stature |
Clinical Pearl: If symptoms persist beyond 2 weeks, it is NOT transient synovitis. Repeat X-ray to look for early Perthes changes (crescent sign).
Surgical Technique
No Surgical Role
Transient Synovitis is a medical condition. There is no role for surgery in the treatment of confirmed transient synovitis.
Role of Surgery is Diagnostic:
- Hip Aspiration: Used to exclude septic arthritis.
- Arthrotomy: Only if septic arthritis is confirmed or strongly suspected (pus on aspiration).
Surgery is otherwise not part of the management algorithm for this self-limiting condition.
Complications
Complications and Pitfalls
| Complication | Risk | Mitigation |
|---|---|---|
| Missed Septic Arthritis | Critical | Apply Kocher criteria strictly |
| Recurrence | 5-15% | Parent education, reassure it's benign |
| Perthes Disease | Unrelated | Follow-up X-ray if symptoms persist over 2 weeks |
| Coxa Magna | Rare | Usually resolves (overgrowth from hyperemia) |
Coxa Magna
Coxa Magna: Mild enlargement of the femoral head can occur due to increased blood flow (hyperemia) from synovitis. It is asymptomatic and usually resolves or persists without consequence. It is NOT Perthes disease.
Follow-Up Protocol
Transient Synovitis Follow-Up
Exclude septic arthritis. Start NSAIDs and rest.
Phone or clinical review. Child should be improving. If worse leads to Red Flag (Re-evaluate for Sepsis/Perthes).
Symptoms should largely resolve. Return to activity as tolerated.
Only needed if symptoms recur or persist. Consider X-ray to exclude Perthes (rare presentation).
Outcomes and Prognosis
Long-Term Outlook
Function: Excellent. 100% return to sports and activities.
Bone Health: No increased risk of osteoarthritis or avascular necrosis (differentiates from Perthes).
Recurrence: Can happen, usually milder. Treat same way (exclude sepsis, NSAIDs).
Evidence Base
Kocher Criteria Original Study
- Established 4 predictors of septic arthritis
- Fever over 38.5, NWB, ESR over 40, WCC over 12k
- 99.6% probability if 4/4 present
- Gold standard for risk stratification
Validation of Kocher Criteria
- Validated Kocher criteria in simpler population
- Predicted probabilities were lower but still significant: 59% for 4 predictors
- Added CRP over 2.0 mg/dL as strong predictor
Caird's 5th Criterion
- Added CRP over 20 mg/L to Kocher criteria
- With 5 predictors, probability 98%
- CRP is independent strong predictor
Transient Synovitis Natural History
- Long-term follow-up of transient synovitis
- No increased risk of Perthes disease
- Recurrence rate 4%
- Benign course confirmed
Ultrasound Efficacy
- Ultrasound highly sensitive for effusion
- Cannot differentiate septic from transient based on appearance alone
- Aspiration is required for differentiation
Viva Scenarios
Practice these scenarios to excel in your viva examination
The Limping Child
"A 4-year-old boy presents with a limp. He had a viral URTI last week. He is afebrile and happy. Examination shows restricted internal rotation. What is your approach?"
The Febrile Limp
"A 3-year-old girl presents refusing to walk. Temp 38.6. CRP 40. WCC 16. Ultrasound shows effusion. How do you manage this patient?"
The Recurrent Limp
"A 6-year-old boy returns 4 weeks after a diagnosis of 'transient synovitis'. He is still limping intermittently. He is afebrile. What is your differential?"
The Perthes Mimic
"A 7-year-old boy presents with a 4-week history of mild groin pain. He has been treated as 'transient synovitis' by his GP but is not improving. X-ray shows 'mild flattening' of the femoral head. Discuss."
MCQ Practice Points
Kocher Probability
Q: A child has 3 out of 4 Kocher criteria. What is the predicted probability of septic arthritis? A: 93%.
- 0 criteria: less than 0.2%
- 1 criterion: 3%
- 2 criteria: 40%
- 3 criteria: 93%
- 4 criteria: 99%
Aspiration Findings
Q: You aspirate a hip. The WCC is 8,000 with 45% PMNs. What is the diagnosis? A: Transient Synovitis. Septic arthritis typically has WCC over 50,000 and over 75% PMNs.
Organism Identification
Q: What is the most common organism causing septic arthritis in the 3-8 year age group? A: Staphylococcus aureus. (Kingella kingae is increasing, especially in younger children under 4 years).
X-ray Findings
Q: What is Waldenstrom's sign? A: Widening of the medial joint space (over 2mm asymmetry) on plain X-ray. It indicates hip effusion (synovitis or septic).
Natural History
Q: Does transient synovitis increase the risk of Perthes disease? A: No. Evidence suggests no link. However, Perthes can initially present similar to transient synovitis.
Australian Context
Guidelines
- RCH Guidelines: Royal Children's Hospital produces the gold standard guidelines used nationally.
- Kocher Criteria: Universal standard in Australian exams.
- Emergency Referral: High risk hips go straight to ED/Ortho On-call.
Epidemiology
- Kingella kingae: Increasing recognition in Australian paediatric population (requires PCR for detection).
- Antibiotic Stewardship: Emphasis on narrow spectrum (Flucloxacillin) unless MRSA risk (rare in community).
Kingella kingae in Australia
Kingella kingae is difficult to culture on standard media. If aspirating a "septic" hip in a young child (under 4y) with negative culture, request PCR for Kingella. This is a frequent exam topic in Australian fellowship exams.
Exam Cheat Sheet
Transient Synovitis Summary
High-Yield Exam Summary
Diagnosis
- •Age 3-8 years, acute limp
- •Diagnosis of Exclusion
- •Must exclude Septic Arthritis
- •Exclude Perthes (X-ray)
Kocher Criteria
- •Fever over 38.5
- •Non-Weight Bearing
- •ESR over 40
- •WCC over 12
- •4/4 = 99% Septic
Management
- •Rest + NSAIDs
- •Observe if Low Risk
- •Aspirate if High Risk
- •NO Antibiotics
Aspiration
- •Septic: WCC over 50k, PMN over 75%
- •Transient: WCC under 15k
- •Culture is definitive
Prognosis
- •Self-limiting (7-10 days)
- •Recurrence 5-15%
- •No long term sequelae
- •Does NOT cause Perthes